Features to preserve Flow Mapping and Flow sequence

[vijayphoenix]

1. These changes help to preserve Flow style Mapping and Flow style Sequence information
2. Fix bugs that occurred after a round-trip
3. Minor code improvements
4. These changes will not become a source for some other errors/problems.

Stats on YAML-test suite

done -- passed: 316 (ev: 32, ev+json: 128, ev+json+yaml: 85, err: 71) / failed: 2 (err: 2, ev:0, json:0, yaml:0, ok:0)

It was also tested on some additional examples which can be found here

[hvr]

here's a first code-review iteration feedback :-)

[hvr]

There's one issue though with this change:

-done -- passed: 316 (ev: 32, ev+json: 93, ev+json+yaml: 120, err: 71) / failed: 2 (err: 2, ev:0, json:0, yaml:0, ok:0)
+done -- passed: 316 (ev: 32, ev+json: 128, ev+json+yaml: 85, err: 71) / failed: 2 (err: 2, ev:0, json:0, yaml:0, ok:0)

i.e. 35 testcases seem to have soft-regressed to not match their YAML reference output anymore; mostly because now we emit the less idiomatic flow-style instead of block-style more often

you'll see this when you diff the output of run-tml before/after your PR

this isn't necessarily a problem with your code but we might need to tweak the run-tml code to have the out-yaml data be normalised like before